In fibrous product processing, such as the process used to make paper, fibers, such as wood fibers and cloth fibers, are separated and mixed with water and, if desired, other ingredients to form a fibrous stock slurry. This slurry is passed through a filter or a screening apparatus, typically referred to as a filter screen, a pressure screen, or a screen cylinder, that filters the slurry to remove from the slurry large particles, such as rocks, stones, metal fragments and the like, unrefined or untreated fiber, improperly sized fiber, as well as other contaminants. After filtering the slurry at least somewhat, the slurry is introduced to a refiner, typically a disc refiner, that grinds or abrades the fibers so they become more frayed or fibrillated. These frayed fibers are beneficial because they interlock with each other during manufacturing of the fiber product to produce a stronger fiber product. For example, where paper is being manufactured, frayed fibers beneficially increase the tensile and tear strength of the paper.
An example of a disc refiner is shown and disclosed in U.S. Pat. No. 5,425,508. The refiner has at least one pair of opposed ridged ring-shaped metal refiner plates that can be formed of pie-shaped refiner plate segments. During operation, one of the refiner plates rotates relative to the other of the refiner plates while the slurry flows under relatively high pressure into a gap between the plates where the plates grind or abrade the fibers.
As one would expect, the refiner plates, or segments of the plates, wear over time which can dramatically decrease the quality of the refining action, i.e. refiner performance, such that the fibers are less frayed than desired. While various parameters of refiner operation are usually monitored, it is believed not heretofore known to collectively monitor and analyze at least some of these parameters while a refiner is operating to attempt to detect or predict when the performance or a refiner plate (or plates) has degraded to the point that that the refiner plate, or segments of the plate, should be replaced. This is because many things other than plate wear can cause the performance of a refiner to at least temporarily decrease making it extremely difficult to detect when plate wear is primarily responsible. Examples of some things that can cause refiner performance to temporarily drop include non-fibrous matter in the stock slurry, a change in stock consistency, knots, and a change in the type or size of fibers being processed.
As a result of the uncertainty of what is responsible for a decrease in refiner quality or performance, refiner plates are typically replaced according to a schedule that mandates replacement after a certain number of hours of operation whether or not replacement is really needed. For example, a typical thermomechanical pulp (TMP) mill may require replacement of the primary plates of a refiner after 2,200 hours of operation and the reject plates of the refiner after 650 hours of operation, regardless of how well the refiner is performing. Because of this, refiners may operate at a less than optimum quality or performance level with plates that needed to be replaced earlier than dictated by the replacement schedule. In other instances, refiner plates that don't need replacing are unnecessarily replaced in accordance with the replacement schedule causing needless downtime and wasting money.
The same is true for filter screens. As is disclosed in U.S. Pat. Nos. 4,954,249; 5,718,826; and 5,626,235, although filter screens can have other configurations, filter screens are often made of perforated and generally cylindrical screen plates, usually referred to as cylinder screens, that are held together by a frame that typically includes retaining rings and tie rods. Typically, two or more such screen plates are disposed end-to-end in a housing, forming a generally cylindrical screen assembly, with the plates held together by the tie rods that extend axially alongside the plates and which pass through the retaining rings.
Each screen plate is formed from metal wire, i.e. wedgewire, or from machined metal sheets. Each screen plate is perforated by holes that typically are slots of a predetermined size that permits objects in the slurry of a size smaller than one of the slots and liquid to pass through the plate. The material retained, referred to as rejects, is drawn away from the surface of the screen plate usually by the head of a moving foil that is located adjacent the screen assembly. The rejects are directed from the filter screen out a discharge port for disposal.
To help ensure that only the slurry is made up of only refined fibers and liquid when it reaches a fiber product processing machine, such as a paper machine, the slurry often passes through several stages of filter screens. Each filter screen typically has correspondingly smaller and smaller holes or slots such that it is possible to limit the size of the fibers that actually arrive at the fiber product-processing machine.
Unfortunately, screen plates wear rather slowly during operation and then rapidly degrade in performance in a rather short time, which makes screen plate failure difficult to predict. To prevent screen plate wear from adversely impacting the fiber product manufacturing process, screen plates are also changed according to a schedule. While the flow rate of the slurry through a particular filter screen can be monitored to provide some sort of an estimate of filter screen efficiency or quality, it is believed heretofore not known to collectively monitor and analyze this and other filter screen operating parameters to attempt to detect or predict when the performance of a screen plate has degraded to the point that it needs to be replaced.
Finally, because fiber product manufacturing processes often operate around the clock seven days a week, an inventory of replacement refiner plates and screen plates are usually kept nearby. Unfortunately, keeping an inventory of these plates takes up valuable and costly space that could be devoted to other more efficient aspects of fiber product production.
Therefore, what is needed is a system and method of managing an inventory of one or both refiner plates and screen plates that minimizes the storage space required at or nearby the fiber product manufacturing plant. What is also needed is a system and method of monitoring refiner performance to detect and preferably indicate when one or more plates of a refiner should be replaced. What is further needed is a system and method of monitoring filter screen performance to detect and preferably indicate when one or more screen plates of a filter screen should be replaced. What is still further needed is a system and method of monitoring refiners and filter screens to detect when one or more plates need to be replaced.